A typical integrated circuit amplifier has a gain that is determined by the ratio of two or more resistance values. The exact value of these resistances and the ratio between them varies because of manufacturing tolerances: it is difficult to achieve a gain accuracy of better than approximately +/−0.1% of the nominal gain. In addition the ratio of the resistances can drift over temperature or time: the exact gain of the amplifier will change from its initial value.
A programmable gain instrumentation amplifier (PGIA) is a multi-stage amplifier with various gain settings that can be implemented by changing the resistances used to implement the gain, e.g., by switching in or out portions of the resistance. Because each gain setting uses different resistance ratios the gain on any particular gain setting can differ from another gain setting by approximately +/−0.1%.
If this amplifier is integrated as a system with an analog to digital converter (ADC), then the apparent transfer function of the converter plus the amplifier will not exactly equal the nominal gain of the PGIA, and will vary over temperature and over time. In addition, converting a constant input signal on different gain settings will produce different effective ADC results, because of the difference in gain error on the different gain settings.
The gain error of the ADC alone can be calibrated to a very high precision, e.g., <+/−0.0015%, by using techniques such as those described in U.S. Pat. No. 5,745,060. However the overall gain error of the system described above will, in general, be dominated by the inaccuracy in the gain of the amplifier. It is therefore desirable to be able to accurately calibrate the gain of an amplifier at any time.
One approach to calibrating the gain of an amplifier is to apply a known/measured voltage to the input of the amplifier, measure the output voltage and adjust the resistance values until the required gain is achieved. This has a number of disadvantages, the principal one being that the applied voltage must be measured accurately, which is not in general possible when the amplifier is installed in a circuit. A one-off calibration is possible, for example a factory calibration, however any subsequent drift in the amplifier gain is not compensated for; therefore the absolute accuracy will degrade over temperature and/or time. In addition, for a programmable gain amplifier, each gain setting must be individually calibrated because of the different resistance values at each gain.
Instead of adjusting the resistance values, the same technique as described above can be used employing a calibration constant stored in non-volatile memory and subsequently used by an ADC or a digital processor to digitally scale the ADC result to compensate for the amplifier gain error. However this also requires an accurately determined input signal, so is not suitable for calibration in the field and calibration values must be calculated and stored for each gain setting of a PGIA.